Wishing for Dark Knight

[Reynhart]

Why not... start a new class? Call it Ravager, make it two-hand swords, and go from there?

Because only one class offers two jobs. We can explore a lot of new mechanics by doing the same to all the existing classes before creating new ones, unless they are really different.
Dark Knight was frequently a Sword/Shield job, I don't think it's necessary to create a class for it.

I believe Dark Knight would branch from a new class that uses a Greatsword or Scythe or go back to the traditional sword and branch from Gladiator; and Dark Knight would be a tank not dps.

I think it's not very useful to give GLA a second tank job. If you wanna tank with a sword and shield, just go for PLD.
A third tank would be far more interesting coming from a class that isn't a tank.

And if something is pretty much standard in the whole FF history, it's that Dark Knight is a damage dealer, whatever weapon you give to him.

Yoshi P said he wanted to make playing a Dark Knight and a Paladin at the same time impossible.

Actually, I'd love to see that, cause I think this game really lacks important choices when building your character. It would be interesting, lorewise, that you have to reject your Paladin duty to be a Dark Knight.
Problem is, all the time spent collecting stuff for your PLD would be for naught, and the backlash would be terrible.